Automatic amplification control



' D. E. HARNETT ETAL Filed Oct. 23. 1937 AUTOMATIC AMPLIFICATION CNTROL-Oct. l5, 1940.

ATTORNEY Patented Oct. l5, 1940 v UNITED STATES AU'roMA'ric Wheeler,tine Corporation,

ABIPLIFICATION CONTROL Daniel E. Harnett, Tuckahoe,

Great Neck, Ni

and Harold A. Y., assigner; to Hazela corporation of DelawareApplication October 23, 1937, Serial No. 170,556

4 Claims.

This invention relates to television-receiving systems and, moreparticularly, to the automatic control of one or more operatingcharacteristics of such systems in accordance with the ampli- 5 tudecharacteristics of received signals.

The invention is especially directed to the provision oi an improvedautomatic amplification control system for television receivers.

In accordance with present television practice,

a transmitted signal comprises a carrier which is modulated duringsuccessive intervals or trace periods by high-frequency and direct(average) components representative of an image being transmitted and ofits average background illumination, and is modulated between the traceperiods or during retrace intervals by synchronizing components whichcorrespond to initiations of successive lines and frames in the scanningof the image.

At the receiver, a beam scan and illuminate a target in a s eries offrames of parallel lines. The synchronizing components of the receivedsignal are utilized to control the scanning apparatus of the receiver soas to syn- 2.3 chronize its operation with that of similar apparatusutilized ing the signal. The intensity of the beam is controlled by thelight modulation components, thereby to reproduce the image.

In certain proposed television systems, the video-frequency modulationcaused by the image is so impressed on the carrier that increases in thecarrier amplitude correspond to increases in Vbrightness of the image,this being termed posi- 3; tive modulation; the background illuminationis transmitted directly, that is, by variation ofthe average carrierintensity during the trace intervals, directly in accordance with thelow-frequency light` variations or steady background; 49 and thesynchronizing impulses are impressed on the carrier in the oppositesense, ordinarily periodically reducing the carrier to zero. Whilesystems utilizing this type oi' signal have, in general, givensatisfactory results, they have been objec- 45 tionable in that it hasbeen difllcult or impossible heretofore to provide satisfactoryautomatic ampliiication control. More particularly, since the averageintensity of the carrier is varied in accordance with backgroundlight-modulation com- 59 ponents, the average intensity cannot beutilized to eilect satisfactory automatic amplification control in theconventional manner which requires some controlling efl'ect dependentsolely upon the average carrier intensity as determined by the 55 powerof the transmitting station, its distance, fading, etc., and independentof the modulation. Nor can the amplitude of the synchronizing impulsesoi such a signal be made readily available for this purpose, as they canin the case of nega- 00 tive modulation where these impulses are transisso deected as to at the transmitter in develop-t mitted on outward peaksof modulation at a relatively xed amplitud since, in positive modulationsystems, the synchronizing impulses are ordiiarily transmitted on inwardpeaks of modala lon.

It is an object oi' the present invention, therefore, to provide in atelevision receiver improved means for automatically controlling one ormore y operating characteristics of the receiver in accordance with anamplitude characteristic oi' the received modulated-carrier signals andindependently of the video-frequency modulation componente. l

It is a further object o! the invention to provide an improved automaticamplification control system iortelevlsion receivers adapted for thereception of positively modulated television signals includingbackground and high-frequency light-modulation components.

In accordance with the invention, a television receiver adapted for thereception of a television signal carrier positively modulated duringtrace periods by background and high-frequency light components andunmodulated by light components during retrace periods comprises acarrierfrequency signal-translating channel in the receiver includingmeans for rectii'ying the. carrier signal translated by the `channelonly during retrace periods for developing a control-bias voltage,together with means for utilizing the developedvoltage to adjust anoperating characteristic of the receiver. s

In a preferred embodiment of the invention,

the control means comprises a signal repeater having an input circuitadapted to -receive the modulated-carrier signal and having an outputcircuit. A rectifier is coupled to the output circuit and means areprovided for developing a periodic wave' having `impulses occurringduring the eld retrace periods of the signal and for utilizing this waveto control the repeater so as to prevent rectication oi' the carrierrexcept during the eld retrace periods. The scanning wave generator ofthe receiver may be used as the source of the periodic impulse wave.'I'he unidirectional voltage developed by the rectier is utilized-tocontrol the amplification in various stages of the system, thereby tomaintain the signal output intensity Within a relatively Vnarrow rangefora lwide range of signal-input intensities. y 2

For a better understanding of the present invention, together with otherand vfurther objects thereof, reference is had to the followingdescription taken in connectionwith the accompanying drawing,and'its'scope will be pointed eut in the appended claims.l i

In the accompanying drawing, Fig. l'is a circuit diagram, partiallyschematic, of al cathoderay tube television-receiving. system including6U circuits embodying the present invention; Fig. 2 is a group of curvesrepresenting certain operating characteristics of the receiver of Fig.1; and Fig. 3 is a diagrammatic illustration of a positively modulatedtelevision carrier wave, to aid in the understanding of the invention.

Referring now more particularly to the drawing, the system therellustrated comprises a receiver of the superheterodyne type including anantenna and ground system IIi-II connected to a radio-frequencyamplifier I2 to which is connected in cascade. in the order named, anoscillator-modulator I3, an intermediate-frequency amplifier I4, adetector I5, a video-frequency amplifier I6, and a cathode-raysignal-reproducing tube I'I. A line-frequency generator I8 and aframe-frequency generator I9 are also coupled to the video-frequencyamplifier and connected to the scanning elements of the cathode-ray tubein the conventional manner. The stages or units Ill-I9, inclusive, mayall be of conventional wellknown construction so that detailedillustrations and descriptions thereof are deemed unnecessary herein.

Referring briefly, however, to the operation of the system describedabove, television signals intercepted by the antenna circuit IIi-II areselected and ampliiied in the radio-frequency ampliiier I2 and suppliedto the oscillator-modulator i3; where they are converted tointermediate-frequency signals which, in turn, aresethe,intermediate-frequency amplifier I4 and delivered to the detectorI5. The modulation components of the signal are derived by the detectorI5 and are supplied to the video-frequency amplifier I6 wherein they areamplied and from which they are supplied in the usual manner to abrilliancy-control electrode of the cathode-ray tube I1 and to thesynchronizing-control elements of the generators IB and I9. Theintensity of the electron beam of the tube I5 is thus modulated orcontrolled in accordance with the video-frequency voltages impressedupon the control electrode of the tube in the usual manner. Saw-toothcurrent or voltage-scanning waves are generated in the lineandframe-frequency generators I8 and I9, which are controlled bysynchronizing-voltage impulses supplied from the video-frequencyamplifier I6 and applied to the scanning elements of the cathode-raytube I1 to produce electricscanning iields, thereby to deflect the rayvertically and horizontally so as to `trace a rectilinear scanningpattern on the screen of the tube and thereby to reproduce thetransmitted picture.

Referring now more particularly to the portion of the system of Fig. 1embodying thepresent invention, for the purpose of developing acontrol-bias voltage there is provided an intermediate carrier-frequencysignal-translating channel including a repeater and a rectifier 2I asshown. The repeater 20 may comprise a vacuum tube of the pentode typehaving an input circuit coupled to the intermediate-frequency arn liiierthrough a coupling transformer 22, 24 and an intermediate-frequencyby-pass condenser 23. A fixed negative bias is provided for the tube 20from a suitable source, for example, a battery 25 connected'between itscathode and ground. Operating potent' als are supplied to the anode andscreen of the tube from a suitable source, indicated as +B and +Sc,respectively. The diode rectifier is coupled to the output circuit ofthe repeater by means of a transformer 21, 29 and is provided with aload circuit `modulated as indicated at M1, Mz,

comprising a resistor 3| and by-pass condenser 3U having a large timeconstant. This time constant should be considerably greater than theframe period which may, for example, be m second, in which event thetime constant may be of the order of 116 second. The negative end of theresistor 3| is connected, by way of a suitable lowpass illter includinga series resistor 32 and shunt condenser 33 and conductor 34, to thecontrol electrodes of one or more vacuum tubes included in the amplifierI2, oscillator-modulator I3, and amplifier Il, as shown.

The frame-frequency generator I9, as stated above, may be ofconventional design and adapted to develop a suitable saw-toothframe-frequency scanning wave. As is well known, however, such agenerator also generally develops at certain points of its circuit aperiodic-impulse wave having impulses at the frame frequency, and thiswave may be taken from a suitable point in the generator. For thepurpose of controlling the amplifier 20 and rectifier 2| to limitrectification of the carrier signal translated thereby to thesignal-retrace periods, therefore, an output circuit of the generatorI9, in which a frame-frequency periodic-impulse wave is developed, isconnected by way of a conductor 2B and the winding 22 to the controlgrid of the tube 20. It is so connected that the impulse voltage isapplied with positive polarity to the grid of the tube 20.

The operation and results obtained by the system just described may bestbe understood by reference to Figs. 2 and 3. The graphs of Fig. 2represent the operating characteristics of the amplifier 20,' theabscissae representing time and the ordinates voltage. The line Arepresents the fixed negative-grid voltage determined by the battery 25.while the line B represents the resultant grid-bias voltage comprisingthe fixed bias, line A, with the frame-frequency impulse Wavesuperimposed thereon. The cutoff bias voltage of the tube is indicatedat C and it will be noted that the normal grid-bias voltage representedby the portions of graph B between the impulses is below cut-off, whilethe impulses carry the tube well above cutoff, permitting thetranslation of signals by the tube 20 to the rectier 2l during theseperiods.

In Fig. 3 there is illustrated the wave form of the modulation envelopeof a television carrier wave of the positively modulated type. The timeY-Yi corresponds to, or at least includes, the duration of one i theimpulses of line B, Fig. 2.

The portions of the wave where the carrier is reduced to zero, certainof which are indicated at L. represent line retrace periods or impulses.The portions between the impulses L correspond to the trace portions ofsuccessive lines and are and Ms by the high-frequency and backgroundlight components, except during the frame retrace periods,

.one off which is indicated at Y-Y1, during which periods the amplitudeof the wave is independent of light modulation. The frame retraceperiods include the line impulses as well as additional broad impulses,as indicated at F, which latter constitute collectively aframe-synchronizing impulse.

It is well known that, with the positive type of modulation. such as isimpressed on the signal shown in Fig. 3, increases in light intensityare represented by increases in the amplitude of the carrier and theaverage intensity of the signal is nents of the image. Thus,

varied in accordance with the low-frequency or direct current backgroundillumination `coxn in the particular portion of the wave shown, at onepoint during the line-trace modulation period represented at M1, theamplitude of the carrier is such that it reaches the white level,whereas this does not occur during the modulation periods represented atMz and M11. Since the average intensity of the carrier is thus varied inaccordance with background illumination variations, these carriervariations cannot be used for the purpose of amplication control. inthis type of wave any synchronizing impulses which could be utilized forthe purpose in question, since the signal is reduced to zero during theline retrace or line-synchronizing impulse periods. However, since thoseportions of the wave occurring during the frame retrace period,indicated at YY1, and between the line retrace periods have substantialamplitude and are not modulated by light components, they afford ameasure of the intensity of the carrier wave independent of lightmodulation which may be utilized automatically to control acharacteristic of the receiversuch as its amplification.

In the operation of the present invention, therefore, a part of themodulated-carrier output of the intermediate-frequency amplifier Il isimpressed upon the control grid of the tube by way of the transformer24, 22. While, as stated above, the battery serves normally to bias thistube below cutoi, as shown in Fig. 2, the impulses of ythe periodicframe-frequency wave, which are shown in line B of Fig. 2 and each ofwhich occurs during a frame retrace period. are impressed upon thecontrol grid of the ampliiier and iserve to reduce the bias and unblockthe tube during at least part of the duration of the frame retraceperiods of the wave.' Hence, only those portions of the wave which areindependent of light modulation are translated to the rectier 2| andrectified therein. A control-bias voltage is thus developed across theresistor 3l and condenser 30 which is'fairly steady and is proportionalto the average intensity of the carrier, independent of lightmodulation. 'I'he lter, comprising the resistor 3l and condenser 30,supplemented by resistor 32 and condenser 33, serves to removeuctuations from the rectied voltage andthe resultant unidirectional-biasvoltage is impressed on the control electrodes o1' one or more of thetubes in the stages I2, I3, and I4 to control the gain oi' these stagesinversely in accordance with variations of the average carrier intensityduring the frame retrace peri Thus, the output-signal intensity of thechannel, including the stages I2, I3, and Il, -is maintained within arelatively narrow range for a wide range of signal-input intensities. Itwill be noted that where the expression wide range oi' signal-inputintensities is employed herein and in the appended claims. it refers tothe intensity variations of the received carrier such as occur duringthe frame retrace periods. independent of light modulation, and not tothe relatively smaller average carrier intensity variations which arecaused by variation oi.' the average background illumination.

It will be appreciated that the .present invention may be utilizedautomatically to control various characteristics ot a televisionreceiver other than its amplification, for example, its selectivity orits timing.

Nor, as stated above, are there.

Thus, while therehas been described what is at present considered to bethe preferred embodiment of the invention, it will be apparent to thoseskilled in the art that various changes and modications ymay be madewithout departing from the invention, and it is, therefore, aimed in theappended claims to cover all such changes and modications as fall withinthe true spirit and scope of the invention.

What is claimed is:

1. A television receiver adapted for the reception of a televisionsignal carrier positivelymodulated during trace periods by backgroundand high-frequency light components and unmodulated by light vcomponentsduring retrace periods comprising, -a carrier-frequencysignal-translating channel in said receiver including means forrectiiying the carrier signal translated by said channel only duringretrace periods for developing a. control-bias voltage, and means forutilizing said voltage to adjust an operating characteristic of thereceive i 2. A television receiver adapted for the reception of atelevision signal carrier positively modulated. during trace periods byIbackground and high-frequency light components and unmodulated bylight`components during retrace periods comprising, means fordeveloping-a .periodic wave having impulses occurring duringsaid"retrace\` periods, a carrier-frequency signal-translating channelin said receiver including means for rectifying the signal translated bysaid channel,

means controlled by said impulse wave for preventing re'ctiilcation ofsaid translated carrier except during said retrace periods, thereby todevelop a control-bias voltage, and means for utilizing said biasvoltage to adjust an operating characteristic of vthe receiver.

3. A television receiver adapted for the reception of a televisionsignal carrier positively modulated during trace periods by backgroundand high-frequency light components and unmodulated by light componentsduring retrace periods comprising, a carrier-frequency repeater havingan output circuit, a rectier coupled to said output circuit, means forcontrolling said repeater for preventing translation of a signal carricrthereby except during said retrace periods, and means coupled to saidrectiiler for utilizing` the products of rectication to control anoperating characteristic oi said receiver.

4. A television receiver adapted i'or the rei ception of a televisionsignal carrier positively modulated during trace periods by backgroundand high-frequency light components and unmodulated by light componentsduring retrace periods comprising. acarrier-frequency ampliiier havingan output circuit,a rectiiler coupled to said output circui ascanning-wave generator for developing periodic impulses during saidretrace periods, .means for controlling said ampli- 'iler includingmeans coupling said generator to said amplliler for preventingtranslation of la carrier signal by said ampliiier except during saidretrace periods, and means coupled to said rectiiier for utilizing theproducts o! rectiiication to control an. operating characteristic ofsaid receiver.

